The present invention relates to a novel styrene-based polymer and a method for the preparation thereof. More particularly, the invention relates to a novel styrene-based polymer of a branched-chain molecular structure obtained by the copolymerization of an aromatic monovinyl compound and a polyfunctional vinyl compound and having, in addition to the excellent impact strength, rigidity and heat resistance, excellent moldability or, in particular, moldability by high-speed injection molding so as to be useful as a molding material of various kinds of household commodities and parts of electric and electronic appliances.
By virtue of the excellent transparency and moldability as well as good electrical and mechanical properties, styrene based polymers are widely used hitherto as a molding material of various kinds of household commodities and parts of electric and electronic appliances. It is a trend in recent years that styrene-based polymers having upgraded properties are required so as to exhibit a good balance between mechanical properties such as impact strength and rigidity and other properties such as heat resistance along with excellent moldability.
It is generally understood that mechanical strengths or, in particular, impact resistance of a styrene-based polymer can be improved when the polymer has an increased average molecular weight. A problem in this way is that the moldability of a styrene based polymer is unavoidably decreased when the polymer has an excessively large molecular weight. Although the moldability of a styrene-based polymer can be improved by compounding the polymer with a plasticizer such as a mineral oil, a plasticized styrene-based polymer has another problem of an unavoidable decrease in the rigidity and heat resistance.
Accordingly, several attempts and proposals have been made in order to simultaneously improve the properties such as mechanical strengths, heat resistance and moldability of a styrene-based polymer including a method in which the fraction of low molecular-weight molecules in the polymer is increased by controlling the polymerization reaction as is disclosed in Japanese Patent Publication 57-30843, a method in which the polymer contains an extremely small amount of ultra-high molecular-weight molecules as is disclosed in Japanese Patent Publication 62-61231 and so on. The former method, however, has a disadvantage that the impact strength of the articles shaped from such a polymer is greatly decreased as the fraction of the low molecular-weight molecules is increased. The latter method is not always applicable to an industrial process because the method involves complicated steps including the preparation of an ultra-high molecular-weight styrene based polymer by the polymerization under conditions different from those in the preparation of the major portion of the styrene-based polymer and blending of a very small amount of the ultra-high molecular-weight polymer with a major amount of the polymer having an ordinary molecular weight. In addition, the improvement in the impact strength is still insufficient in the styrene-based polymer obtained by such a blending method.
Alternatively, a method has been proposed for the simultaneous improvement in the impact strength and mold-ability of a styrene-based polymer by the copolymerization of an aromatic monovinyl compound, e.g., styrene, and a polyfunctional vinyl compound so as to give a copolymer having a broad molecular weight distribution. This method, however, is not free from the problem that, due to the microscopic distribution of crosslinked or gelled molecules, the polymer is not satisfactory in respect of the rigidity, high-speed moldability and heat resistance.
Thus, no method is hitherto known to provide a styrene-based polymer having a good balance among the properties such as impact strength, rigidity, heat resistance and the like along with excellent moldability suitable for high-speed injection molding.